Rispenzepine is a novel antimuscarinic compound with a preferential action at M1, and M3 receptor subtypes. The presence of muscarinic autoreceptors in human and guinea pig trachea is investigated by comparing the effects of the muscarinic receptor antagonists Pirenzepine (M1), Methoctramine (M2), 4-DAMP (M3), and Rispenzepine (M1/M3) on cholinergic neural contractile responses evoked by electrical field stimulation (EFS) and [3H]ACh release. The M1, M1/M3, or M3 antagonists inhibit the EFS-evoked cholinergic contractile response in a concentration-dependent manner (4-DAMP > Rispenzepine > Pirenzepine), whereas Methoctramine facilitates this response at low concentrations (<3 μM). In ACh release studies, the M3 antagonist has no significant effect, whereas Pirenzepine, Methoctramine, and Rispenzepine significantly increase ACh release in guinea pig trachea. Rispenzepine almost completely inhibits cholinergic, contractile responses at 0.3 μM (92.7±6.2% inhibition, n=6, p<0.05; pD2 value of 7.31±0.15) .